1. Technical Field
The present invention relates generally to detecting electromagnetic induction sensing and processing, and in particular, to a mobile and compact electromagnetic induction sensing system that may be utilized in overhead detection applications for detecting small discrete objects such as landmines.
2. Description of the Related Art
Electromagnetic induction (EMI) detection techniques are applied in a variety of contexts including non-destructive testing of manufactured objects, mineral exploration, treasure hunting, security checkpoints, and detection of mines and unexploded ordinance (UXO). The hand-held metal detector used for treasure hunting as well as landmine sweeping is a familiar example of an EMI sensor. The operation of EMI sensors is based on principles of electromagnetic induction in which one or more inductor coils are utilized to interact with buried or otherwise hidden metallic elements or objects. Specifically, an EMI sensor includes a transmitter coil that emits a primary magnetic field into the surrounding environment (a ground surface or container, for example). The primary magnetic field induces eddy currents within nearby electromagnetic reactive elements, resulting in emission of a secondary magnetic field which is measured by the EMI sensor as an electric potential or electromotive force across a receiver coil. For ease of reference, the terms “EMI sensor,” “EMI detector,” and “metal detector” are utilized herein synonymously throughout.
The metal detectors used for mine and UXO detection are remarkably sensitive, capable of detecting buried objects containing less than a gram of metal. Therefore, even with the advent and increasing utilization of low metallic content mines and other ordinance, EMI sensors remain a staple in the field of mine and UXO detection. The basic objectives of detection using EMI sensors include obtaining the highest probability of detection (Pd) and the lowest false alarm rate. Furthermore, for wide area assessment there is an increasing interest in the speed at which an area can be covered.
EMI mine and UXO detection may be deployed using hand-held, ground based vehicle-mounted or airborne detectors. Vehicular-mounted EMI detection provides faster ground coverage but is limited to vehicle-accessible terrain. Although effective for reliable detection over terrain inaccessible by vehicle, hand-held EMI sensors pose the highest risk of human injury of any of the methods and are ineffective for providing rapid assessments of vast areas such as is often encountered during or after wartime conditions in which minefields may span hundreds or thousands of square miles. Furthermore, some terrain conditions such as mine or UXO contaminated underwater or wetland environments may preclude use of either vehicle-mounted or hand-held EMI detection.
Airborne electromagnetic induction (A-EMI) in which the detector is mounted to the underside of an aircraft, such as a helicopter or fixed-wing airplane, is effective for determining apparent conductivity in near-surface geophysical studies. A-EMI induction systems are currently used in many areas of environmental and geophysical exploration including detection of mineral deposits, saltwater intrusion studies, and petroleum exploration. Conventional A-EMI detectors provide much faster ground coverage but have a lower Pd when used for individual landmine or UXO detection than the other two methods. A problem with conventional A-EMI sensing methods when applied to mine and UXO detection is that the altitude distance between the detector and the ground surface results in a lateral “footprint” of the receiver coil being on the order of tens of meters. Because the sensor response is averaged over the receiver footprint, adequate resolution is not provided for relatively small conductive or dielectric objects such as landmines. Furthermore, A-EMI systems are physically unable to gain close proximity to the ground in order to reduce the size of the footprint and therefore increase resolution.
It can therefore be appreciated that a need exists for an improved A-EMI sensing apparatus and system that enables rapid, scalable detection and imaging to provide accurate and rapid detection of mines, UXO and the like. The present invention addresses such a need.